The present invention relates to an apparatus for separating a dispersed liquid phase from a continuous liquid phase by electrostatic coalescence and to a process in which use is made of such an apparatus, in particular a process for dehydrating hydrocarbon liquid emulsions in which such an apparatus is employed.
The term "coalescence" may be defined as the coming together of small droplets of liquid to form larger droplets permitting easier and more rapid phase separation. One of the methods for achieving coalescence of liquid droplets comprises subjecting a liquid emulsion to a suitable electric field of sufficient intensity to cause the dispersed liquid phase to coalesce. It will be understood that such an electrical treatment is only suitable if the dispersed liquid phase is relatively conductive and the continuous liquid phase is relatively non-conductive. The technique of electrostatic coalescence is well known and is widely applied, in particular in processes for dehydrating hydrocarbon liquid emulsions, such as crude oil desalted by washing with fresh water.
A large variety of different types of electrostatic separators have been proposed in the past and are commercially applied. Some of these separators are designed to produce uniform electric fields for effecting droplet-coalescence, whereas other separators are provided with internals to produce a non-uniform electric field for generating droplet-coalescence. In a uniform electric field the lines of forces are parallel to one another and the field strength is constant throughout the space between the electrodes. In a non-uniform electric field, however, the lines of forces are not parallel to one another and the field strength will therefore be a function of the location in the field.
The known electrostatic separators for liquid/liquid separation are normally equipped with electrodes having such a configuration that during operation uniform electric fields are generated. U.S. Pat. No. 3,582,527, for example, describes a system for resolving an emulsion by electrostatic coalescence, in which a vessel is provided with electrode means extending over the entire cross-section of the vessel to guarantee that the emulsion is completely subjected to a uniform electric field. Most electrostatic separators used for treating liquid emulsions is of the so-called uniform electric field type.
The electrostatic separators for separating solids from a continuous liquid phase or a gas phase, on the contrary, are normally provided with electrode means enabling the generation of non-uniform electric fields. Although the latter type of electric fields are less prone to maloperation due to short-circuiting, a problem occurs when separators of the non-uniform field type are used for separating liquid emulsions.
U. S. Pat. No. 3,577,336 is one of the relatively few publications describing an electric treater for liquid emulsions provided with rod-like electrodes in combination with electrodes surfaces that are substantially planar to generate non-uniform electric fields.